The invention relates to an electron beam device such as a cathode ray tube (CRT) comprising an evacuated envelope including an electron gun comprising electron-beam means for producing in operation at least one electron beam directed to a surface and a resistive focusing lens structure for producing a focusing lens field for focusing said at least one electron beam to a spot on said surface, said focusing lens structure comprises a resistive material and has supply means to supply a voltage to said focusing lens structure.
The invention also relates to a method of providing a resistive focusing lens structure for an electron beam device comprising electron-beam means for producing at least one electron beam directed to a surface, said focusing lens structure being provided on an inside surface of a glass member to be positioned around the path of said at least one electron beam.
A conventional electron beam device comprises an electron gun which, generally, comprises a number of sheet metal parts (grids) held together by glass rods. The grids are at different voltages to produce lenses for focusing the electron beams. A major shortcoming of a conventional gun is the spherical aberration of the main lens, causing rays passing through the outer zones of the lens to be refracted more than the paraxial rays. This leads to increased spot sizes on the surface on which the electron beam is focused; said surface in a CRT is usually called a screen. The focusing efficiency can be improved by increasing the number of metal grids, but this is preferably done without increasing the number of high-voltage leads though the envelope. A solution to this problem was found by interconnecting extra grids by a resistive voltage divider which supplies the high voltages for the extra grids. A better solution, however, is to integrate the grids and the voltage divider in a single part by using a resistive focusing lens structure. A resistive focusing lens structure comprises an elongated tubular resistive element patterned to create an electron lens when a voltage difference or voltage differences are applied over the resistive element. In some known electron beam devices a resistive focusing lens structure is formed by an elongated tubular member which is coated on the inside with a high-ohmic resistive layer patterned to create an efficient electron lens. The electric field created when a current is passed through the high-ohmic resistive layer acts as an efficient focusing lens. In addition, resistive focusing lens structures enable the realization of lenses with low spherical aberration.
Electron beam devices of the type mentioned above are known in the art and are described in, for example, Vrijssen, U.S. Pat. No. 4,713,879, Spanjer et al., U.S. Pat. No. 4,827,184, Vrijssen et al., U.S. Pat. No. 4,857,797, and Vrijssen et al., European Patent Application EP-A 513,909.
Although the known electron beam devices offer in respect of conventional electron beam devices a much improved focusing ability, they have as yet not been made in mass production. The inventors have realized that a major problem is caused hereby that, in order to make full and efficient use of the improved focusing ability, the electric focusing field generated by the resistive focusing lens structure has to comply with high standards of reproducibility. The electric fields generated by the resistive focusing lens structure should be reproducible in time and between different focusing lens structures.